1. Field of the Invention
This invention is directed to microelectronic devices and, in particular, to a method for repairing defects in high density multilayer ceramic chip carriers using polyimide compositions for the repair process to reclaim product and reduce production cost and to assure product reliability in long term functional performance.
2. Description of Related Art
In the fabrication of high density electronic modules based on multilayer glass ceramic chip carriers, it is generally observed that there is significant loss of product due to ceramic surface damage and internal electrical defects occurring during the manufacturing operations resulting in lower yield which adds to the production cost of the product. Typical surface defects, for example, at the seal band region of non-hermetic and flange areas in hermetic chip carriers caused by handling and test procedures are of the type such as ceramic depressions, voids, cracks or chipped ceramic. For the seal integrity in the top surface protective cap attachment, it is critical that the top surface metal (TSM) flange area be free of such defects in order to assure hermetic sealing to protect the silicon devices from any detrimental effect of environmental exposure during the long-term functional performance of the electronic device. The electrical defects can be internal shorts in metallurgy within layers of multilayer ceramic substrate and surface defects due to manufacturing design errors and plating defects which are sometimes detected at testing after complete fabrication.
A practical and reliable method of defect repair in fully assembled high-density ceramic modules is required to restore/assure long-term functional performance and to recover/reclaim high performance glass ceramic products to provide cost reduction and waste minimization. Without having a workable repair method, many high cost glass ceramic substrates have to be discarded due to internal defects and/or surface defects, adding to the overall product manufacturing cost, waste, and waste disposal cost.
For repair of surface damage and structure defects within layers in assembled modules, passivation with a suitable organic insulator can be used in repair processes for engineering change, exposed metal defects repair and functional repairs to provide protection from solvents, and other materials used in bond and assembly (BAT) processes. For designing and implementing a cost effective repair method, it is necessary that the polymer insulator material(s) have the requisite characteristics for the application, particularly, compatibility with high temperature chip join solder reflow conditions, rosin flux exposure, flux residue solvent cleaning process, good mechanical properties, low moisture absorption, and good adhesion to ceramic and bonding metals to assure functional reliability and its durability in long term use of the device. In addition to high thermal stability required for high temperature device joining by solder reflow requiring 350-365° C. peak temperature for 97/3 lead/tin alloy (97% Pb/3% Sn), the material used for passivation in defect repair is also required to have good adhesion to all contacting surfaces. It is also desirable that the polymer materials used for repair have compatibility with lead-free solder joining of silicon devices to chip carriers up to 260-270° C. peak temperature. Other requirements for the insulator material in defect repair processes is the ability to fill vias up to at least 15 μm deep etched into the ceramic without introducing voids as well as the ability to repair ceramic defects as chipped ceramic by filling with a polymer composition that shows no blistering or voids when it is subjected to thermal curing to form thermally stable polyimide structure. Yet another requirement for the insulator is the ability to write passivation lines with an autodispense tool, for example, a Pen Writer tool, for high throughput repair processes. For automated writing or manual dispense, it is preferred that the material has good wetting of ceramic and metal surfaces, shows no stringiness, be gel-free, maintain line shape and dimension after writing, have good flow properties suitable for automated line writing and via fill without introducing voids and have low shrinkage upon curing.
A number of polyimide compositions for use as passivation coatings and device protection are commercially available. Solder mask compositions are used to cover the selected areas of printed circuit board (PCB) circuitry to prevent solder bridging. None of these materials, however, are found useful for applications requiring defect repair in high-density electronic modules due to incompatibility with the ceramic product repair processes. Automated writing of passivation coating lines with a Pen Writer presents another challenge where continuous writing of line patterns, without the pen tip clogging or wicking, is necessary for high throughput, uniform thickness, and high quality cured films.
U.S. Pat. No. 6,262,390, assigned to the assignee of the subject invention, discloses a method to repair Aluminum Nitride (AlN) substrates using an insulating polyimide derived from reacting stoichiometric amounts of non-rigid dianhydrides and aromatic amines having at least one hexafluoroisopropylidene functionality.
U.S. Pat. No. 5,310,863, assigned to the assignee of the subject invention, discloses novel polyimide materials for use as an interlevel dielectric and passivation layer in the fabrication of thin film structures in ceramic chip carriers for high end applications, the polyimides being derived from rigid or semi-rigid aromatic dianhydrides and linear or non-coplanar aromatic diamines carrying a —CF3 group.
U.S. Pat. No. 5,115,090, assigned to the assignee of the subject invention, is directed to low thermal expansion polyimides derived from rigid-rod and linear-planar dianhydrides and diamines using stoichiometry offset method to generate materials with varying molecular weight.
The high temperature stable polyimides of the prior art derived from linear-rigid-rod and planar precursors generally have low solids content, up to 13-14% (wt. %) in n-methylpyrrolidone (NMP) and thus higher shrinkage upon curing. These polymers invariably require substrate surface preparation for the necessary interface integrity with ceramic, copper and other contacting metals. The cured coatings have glass transition temperature (Tg) above 300° C., typically between 350 and 400° C., have high modulus and tensile strength, which are highly desirable properties for use as interlevel dielectric layer in high density thin film structures.
Selected candidate materials described in the above patents can be used for the purpose of the subject application by using surface modification, deposition of interface metallurgy for adhesion to Cu, typically a thin layer of Cr on Cu prior to polymer application, and multiple coatings for thick coatings. It is, however, preferred that polyimide compositions for the defect repair purposes have higher solids content, up to 25% (wt. %), or more, so as to have lower shrinkage upon curing, have flexibility and flowability and preferably showing thermoplastic behavior at less than 300° C. temperature to allow gap filling and planarization by melt flow of cured coatings, and not necessarily requiring an adhesion promoter for adhesion to ceramic, copper, and other metals.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of this invention to provide a method for repairing defects in high density ceramic chip carriers by using improved/modified polyimide compositions for the defect repair processes for product yield loss recovery, production cost reduction, and product reliability assurance in long term functional performance.
Another object is to provide polyimide insulator compositions with special properties for electronic product defect repair having compatibility with automated polymer line patterns deposition with a Pen Writer, having good adhesion to a metal such as Cu, and having compatibility with bond and assembly processes for chip joining using high temperature Pb/Sn solder, lead-free solder joining, flux residue solvent cleaning, and plating solutions for terminal metallurgy.
It is also an object of this invention to provide polyimide materials for passivation or micropassivation for the purpose of repairing under the device buried defects in electronic modules which have solids content in the range about 12 to about 35%, preferably about 15 to about 25% (wt. %) in a high boiling solvent, preferably NMP, and have viscosity and flow properties suitable for pen writing with an automated writing tool and which show low shrinkage and no significant blistering upon curing, and the cured coatings have low moisture uptake.
It is another object of the present invention to provide a method for repairing microelectronic substrates particularly glass ceramic substrates.
It is another object of the present invention to provide a method of repairing ceramic substrates with a material which is compatible with bond and assembly processing which includes high temperature chip join solder reflow conditions, rosin flux exposure, flux residue cleaning solvents and processes, and have good mechanical properties.
A further object of the invention is to provide a method of repairing ceramic substrates with a material, which has good adhesion to all contacting surfaces and has low moisture uptake.
It is yet another object of the present invention to provide a method of repairing ceramic substrates with a material which has good flow properties for filling ceramic via holes and metal trenches without introducing voids and which has low shrinkage upon curing.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.